1. Field of the Invention
The present invention relates to a joint structure for joining an outboard engine to boat hull, and more particularly to such a joint structure which has an elastic mount.
2. Description of the Prior Art
Generally, an outboard engine assembly comprises a vertically arranged engine with the crankshaft extending vertically. The engine is fastened to an extension case of an outboard engine assembly body. With the engine thus fastened, a crankshaft chamber housing a crankshaft is positioned forwardly, and a cylinder defining a combustion chamber and a cylinder head are positioned rearwardly. A vertical shaft for transmitting propulsive forces from the engine to a propeller extends through the extension case.
Joint structures for joining such outboard engine assemblies to boat hulls are disclosed in Japanese Laid-Open Patent Publication No. 56-116597, and U.S. Pat. Nos. 2,916,007, 3,002,489, and 3,599,594, for example.
According to the joint structures shown in Japanese Laid-Open Patent Publication No. 56-116597 and U.S. Pat. No. 2,916,007, a rubber member is interposed between inner and outer collars, and the inner collar is coupled to one member and the outer coller to another member. The joint structure disclosed in U.S. Pat. No. 3,002,489 includes three rubber members in each of upper and lower regions between members to be joined to each other. In the joint structure shown in U.S. Pat. No. 3,599,594, a crossbar extends through one member and has opposite ends fixed to another member, and a tubular rubber member is wound around the crossbar with a rubber strip in a portion of the other member.
Joint structures to be used between boat hulls and outboard engine assemblies are required to transmit propulsive forces efficiently from the outboard engine to the boat hull without power loss and also to prevent vibrations from being transmitted from the engine to the boat hull. While the elastic elements are employed in the joints for vibration absorption in the prior joint structures, no sufficient vibration absorbing capabilities have been achieved because of the contradictory natures of the efficient transmission of propulsive forces and the prevention of vibration transmission.
It has been considerably cumbersome to establish spring characteristics of the elastic elements. Other problems of the conventional joint structures are as follows:
In the conventional outboard engine assemblies, vibrations are produced about an axis, i.e., a torque roll axis, due to reactive forces generated in response to the drive torque of the engine, the torque roll axis being positioned substantially in the vicinity of the vertical shaft or slightly rearwardly of the vertical shaft. In the earlier joint structures, the position of the upper or lower mount is limited by the peripheral wall of the extension case which is spaced from the vertical shaft to avoid physical interference therewith, and is spaced from the torque roll axis. Such a configuration increases the rigidity of the elastic elements of rubber, making it difficult to reach the simultaneous achievement of supporting the weight of the outboard engine, giving a preferable steering feeling, and preventing the transmission of vibration.